Mineral wool is a fiber made from natural or synthetic minerals or metal oxides. Industrial applications of mineral wool include thermal insulation, filtration, padding, and soundproofing (e.g., ceiling tiles).
Mineral wool is comprised of fibers of inorganic raw materials. Mineral wool is a term broadly applied to various related vitreous products. In general, mineral wool is a fiberglass-like material composed of very fine, interlaced mineral fibers, somewhat similar in appearance to loose wool. It is composed primarily of silicates of calcium and aluminum, chromium, titanium, and zirconium. Typically, mineral wool is produced from natural rock or slag. Slag is a term broadly applied to refer to waste products of the primary metal and foundry industries, including deposits from the furnace lining charge impurities, ash from fuel, and fluxes used to clean the furnace and remove impurities. Generally speaking, although mineral fibers have an appearance that is similar to that of glass fibers, their chemical composition is significantly different from that of glass fibers due to the high content of iron and calcium and magnesium and a relatively low proportion of silicon dioxide and aluminum.
Slags are classified as either “acid” (i.e. high silicate) slags or “basic” slags, depending upon the relative quantities of acidic and basic sub-components. For example, typical acid slags contain between forty and fifty percent (40.0 to 50.0%) of acidic subcomponents, such as silicon dioxide (SiO2), from twenty-five to forty-five percent (25.0 to 45.0%) of basic sub-components, such as the oxides of calcium (CaO) and magnesium (MgO), and from ten to twenty percent (10 to 20%) of alumina Al2O3. A typical basic slag that is used to reduce metals comprises between twenty-five and fifty percent (25.0 to 50.0%) acidic subcomponents such as silicon dioxide (SiO2) and alumina (Al2O3), and a relatively high percentage, between thirty-four and fifty percent (34.0 to 50.0%) basic subcomponents, such as the oxides of calcium (CaO) and magnesium (MgO). Magnesium may be added to increase the basicity of the slag. Basicity is the tool used to determine the metal quality using basic slag. Basicity is calculated as follows: (CaO+MgO)/(Al2O3+SiO2). The basicity of typical basic slags ranges between 0.93 and 1.9.
Mineral wool is often classified according to the inorganic raw materials used in its production. For example, Rock Wool is produced from combinations of natural rocks and/or minerals. Slag Wool comprises a composition of iron, copper and lead slags typically removed from blast furnaces, and may contain some fluxing materials. Refractory (high-temperature) or “Certa” wools may be made from the oxides of aluminum, chromium, zirconium, or titanium and silica sand. Further subclassifications of these products relate to the quality or purity of the wool. For example, slag wool is subclassified for purity according to color: black, gray, and white wools are available. A tool for determining the quality of mineral wool produced from a slag charge is the acid-to-base ratio (A/B). The formula for determining A/B is (Al2O3+SiO2)/(CaO+MgO). As described in U.S. Pat. No. 5,496,392, in a typical mineral wool derived from cupola slag, the acid-to-base ratio ranges between 0.74 and 2.316.
Further, as described in U.S. Pat. No. 8,697,588 issued to USG Interiors, Inc, in conventional mineral wool, components are typically selected and proportioned in a way that maintains a target acid-to-base (A/B) ratio of the constituent fibers. The A/B ratio is a key parameter because it corresponds to fiber solubility, or how readily the fibers dissolve in bodily fluids, such as blood and saliva. This is an important characteristic for reasons having to do with human health, because these materials are likely to come into contact with humans as a result of their use in building construction materials.
Mineral wool plays a role in acoustical ceiling tile performance. Standard acoustical ceiling tiles have a noise reduction coefficient (NRC) greater than 0.50 and up to 0.70 and ceiling attenuation class (CAC) values of less than 35. Sound absorption is typically measured by its Noise Reduction Coefficient (“NRC”) as described in ASTM C423. The NRC value is an average of four sound absorption coefficients of the particular surface at frequencies of 250 HZ, 500 HZ, 1000 HZ and 2000 HZ, which cover the range of typical human speech. NRC is represented by a number between 0 and 1.00, which indicates the fraction of sound reaching the panel that is absorbed. An acoustical panel with an NRC value of 0.60 absorbs 60% of the sound that strikes it and deflects 40% of the sound. Another test method is estimated NRC (“eNRC”), which uses an impedance tube as described in ASTM C384. The ability to reduce sound transmission is measured by the values of Ceiling Attenuation Class (“CAC”) as described in ASTM E1414. CAC value is measured in decibels (“dB”), and represents the amount of sound reduction when sound is transmitted through the material. For example, an acoustical panel with a CAC of 40 reduces transmitted sound by 40 decibels. Similarly, sound transmission reduction can also be measured by its Sound Transmission Class (“STC”) as described in ASTM E413 and E90. For example, a panel with an STC value of 40 reduces transmitted sound by 40 decibels.